A medicine or remedy that acts quickly after administration; a fast-acting remedy.
頭痛がひどかったので、即効薬を飲んだ。
My headache was severe, so I took a fast-acting medicine.
この軟膏は即効薬ではないが、継続して効く。
This ointment isn't a fast-acting remedy, but it works with continued use.
花粉症の急な症状には即効薬が便利だ。
Fast-acting medicines are useful for sudden hay fever symptoms.